An ultrasonic sensor which measures the distance to a target object by using ultrasonic waves is requested to have sharp directivity. To improve the directivity of the ultrasonic sensor, the vibration mode of a vibration surface has been devised in the past.
For example, ultrasonic vibration devices each serving as an on-vehicle ultrasonic sensor for a back sonar are disclosed in Patent Documents 1 and 2. In both of the ultrasonic vibration devices of Patent Documents 1 and 2, a piezoelectric element is bonded to a cylindrical case with a bottom, and the interior of the case is provided with a sound-absorbing member for absorbing rear sound and is filled with an elastic damping member to attenuate vibration.
The ultrasonic vibration device of Patent Document 1 is configured to use a case hollowed out into an elliptical shape having a long axis and a short axis, and obtain the anisotropy of radiating acoustic waves in accordance with vibration with nodes occurring in a bottom portion of the case.
FIG. 1 is a cross-sectional view of the ultrasonic vibration device disclosed in Patent Document 2. An ultrasonic vibration device 10 includes a cylindrical cap body 12 with a bottom. A piezoelectric element 14 is bonded to a bottom surface portion 12a inside the cap body 12 by a conductive adhesive agent or the like. An inner frame 16 higher in acoustic impedance than the cap body 12 is fit inside the cap body 12. The inner frame 16 is fit to be in close contact with a side surface portion 12b including an end portion of the cap body 12. A side surface of the inner frame 16 is formed with through-holes 22. Wiring members 24a and 24b are connected to the piezoelectric element 14 and the inner frame 16, respectively. The interior of the inner frame 16 is filled with a sound-absorbing member 26 and a damping member 28. The damping member 28 is in direct contact with the side surface portion 12a of the cap body 12 through the through-holes 22 of the inner frame 16.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-284896
Patent Document 2: Pamphlet of International Publication No. WO 2007/069609